1. Field of the Invention
The present invention relates to a thermal printing head and, more particularly, to a thermal printing head suitable for producing half-tone prints.
2. Prior Art
Printers employing a thermal transfer printing method are widely used as printing systems in office automation equipment such as facsimile machines and word processors or in producing hard copies of video images because printers of this type are inexpensive, they generate less noise during operation, and they can be made compact. In order to obtain good printed characters or images with a printer employing a thermal transfer printing method, the thermal printing head and the printing paper must be brought into close contact with each other so that the heat generated by the head is accurately transferred to the printing paper. For this purpose, Japanese Patent Laid-Open No. 159176/1981 discloses a conventional thermal printing head in which convex glazed layers are formed on the surface of a substrate, band-shaped heating elements are formed on the glazed layers, and conductive layers are provided adjacent to and on either side of heat generating portions of the heating elements. Thus, according to the above-mentioned disclosure, the heat generating portions of the thermal printing head are formed with a generally convex shape, thereby attaining good contact between the head and the printing paper or ink sheet.
The conductive layers provided on either side of the heat generating portion of the head are formed of thin films. These thin films, however, have a thickness of 1 to several .mu.m. As a result, the heat generating portions are each positioned at the bottom of a valley portion formed between parts of a conductive layer, resulting in an increase in the distance between the heat generating portions of the thermal printing head and the printing paper. Accordingly, even when the temperature of the heat generating portions of the head was precisely controlled, it has been difficult to achieve an image quality properly reflecting that temperature control. In addition, since the conductive layers provided on either side of the heat generating portions usually have good thermal conductivity, the heat of the heat generating portions may also be transferred to the conductive layers. At such a time, the printing paper may be heated over an area wider than the dimensions of the heat generating portions, resulting in the printing of dots with a low density. The heat generating portions interposed between the parts of the conductive layers also raise a problem in which they tend to trap fine particles of paper.